March 25, 2004 News: Arlington Marathon Is Back On   Arlington Marathon Is Back On First it was on, then it was off, and now it appears that the inaugural Potomac River Run, a marathon in [Alexandria], Va., is back on. Jay Wind, the organizer of the May 2 event, announced this week that the 26.2 miler is a go. The National Park Service has issued permits for the 200-person race, which will bring runners out and back along the Mount Vernon Trail in [Alexandria]. There is both a marathon and a half-marathon division, and the races will start and finish at Belle Haven Park. For more information on the event, see www.pvtc.org/marathon.html.